this is a good topic, I used to know someone who was research this, not sure where he went this. Yeah so your head hair does different from you back and chest hair. For example a man who is going bald due to various known causes will still have a chest and back full of hair.

Assuming the difference of interest is the distribution of hair over the human body. It also differs in texture... pubic hair is generally stiffer than head hair for eg. OTOH: we may also ask why we may expect that it shouldn't be different on different parts of the body.

The example was given for monkeys - however, you will see that monkeys usually have shorter fur over their faces than they do elsewhere and some have quite long fur down their backs. Apes tend to be bald on their faces. etc.

The human hair is markedly different from other mammals though - we are strikingly bald with no fur at all and a variety of different kinds of hair. IFAIK: the details are something of a topic for ongoing research - the human fossil record does not, so far, contain hair or skin so we have no direct knowledge. The scientific american article linked to discusses this and relates some of the ideas.

I tend to be cautious about this topic because creationists have been known to use "evolution does not explain human hair therefore humans were made by God".

I tend to be cautious about this topic because creationists have been known to use "evolution does not explain human hair therefore humans were made by God".

I've never run into this, but I suppose they'll seize on any possible issue.

Now that the OP asks, I too am curious. The difference that intrigues me is that head and beard hair grow constantly while body hair ( and eyebrows and eye lashes) stops growing at a certain length. Head and beard hair will automatically become unwieldy after a few years and be a liability unless something is done about it.

I seem to attract them - and UFO nuts, and perpetual motion enthusiasts, and so on and on. You can see how that would warp the way you answer questions.

How the length of human hair, particularly on the head, is regulated has been the subject of intense study. There is a huge market for baldness cures for eg. So there is no end of data on these mechanisms. For instance, iirc, you can switch hair growth on and off with an enzyme; baldness and hair thinning in old age has a genetic component (even old apes can get thinning hair all over.) But it's not something I've personally studied in great detail.

The evolutionary mechanism that ends up with the (relative to other mammals) baldness in humans is something of a mystery. I'd bet on it being a side effect of something else... perhaps the mechanism for something else like language or self-awareness also stops your hair growing but it's not a disadvantage because we can wear clothes? Though judging by the way many people seem to prefer hairlessness in their partners, it may be a secondary sexual characteristic.

The difference that intrigues me is that head and beard hair grow constantly while body hair ( and eyebrows and eye lashes) stops growing at a certain length.

I had asked my physician the same question a very long time ago, "Why does hair on my arm grow to a specific length, and the hair on my head keep growing?" He said that some hair on the body falls out after growing for a given period of time which is why it always seems to be a specific length. It never stops growing, it just falls out.

I'm not sure if this is the reason, but like I said, that's what my physician told me. Maybe someone can confirm/deny?

I had asked my physician the same question a very long time ago, "Why does hair on my arm grow to a specific length, and the hair on my head keep growing?" He said that some hair on the body falls out after growing for a given period of time which is why it always seems to be a specific length. It never stops growing, it just falls out.

I'm not sure if this is the reason, but like I said, that's what my physician told me. Maybe someone can confirm/deny?

I think he must have been making that up on the spot. I think body hair lingers in place without growing a long time before it falls out.

As far as I know, humans are the only animals whose head hair keeps growing indefinitely. This must have been a liability for early humans. Eventually, you have to take action to keep it out of the way. It's not an advantage in the cave or on the savannah, but we can assume it didn't become an obstacle to living to the age of procreation.

Maybe the hair of some other animals does keep growing, though. Maybe a wild sheep would become so wool bound it would eventually die from immobility. I don't know.

this is a good topic, I used to know someone who was research this, not sure where he went this. Yeah so your head hair does different from you back and chest hair. For example a man who is going bald due to various known causes will still have a chest and back full of hair.

Yes, but I'm not sure if that has to do with the type of hair, balding men usually only loses hair on the top and the front of the head, they still have hair right above the ears and in the back. And when they with hair transplantation move hair from the back of the head to the front, the hair survives so it seems to me like there is some difference in the back head and front head hair. Thats kind of strange too, that some of the head hair is more resisting to balding than the other.

I did a little research on this. Seems the physician I mentioned had the right idea after all. Hair grows in a 3 part cycle, and the cycle can vary depending on where on your body it grows. I remember as a kid, shaving hair off my arm (don't ask) and finding it always grew back to the same length. The reason is as the physician mentions, it follows a cycle and then falls out. Note that hair doesn't have any signals running up and down its length to tell it when its at the right length to "stop growing" so to speak, because there is no such thing as a hair that knows how long it is. Hair has no way of determining how long it is so it can't grow to a certain length and stop. Instead, it grows for a certain amount of time and stops and then falls out.

Hair grows and falls out in a continuous three-part cycle. The three parts of the hair cycle are the growth (anagen) stage, the transition (catagen) stage, and the resting (telogen) stage. The hair falls out during the resting stage, which lasts about two to three months. Fortunately, not all of the hair is in the same part of the cycle at the same time. At any one time, only about 10% of the hair is in the resting stage. Of the approximately 100,000 hairs on the average head, it is normal to lose about 100 hairs per day.

The typical growth cycle lasts two to six years, during which time hair grows about half-an-inch per month. At the end of the growth stage it enters the transition stage, which lasts two to three weeks. During this stage, hair growth stops and the hair root shortens and shrinks. The final phase is the resting phase, when hair falls out.

I did a little research on this. Seems the physician I mentioned had the right idea after all. Hair grows in a 3 part cycle, and the cycle can vary depending on where on your body it grows. I remember as a kid, shaving hair off my arm (don't ask) and finding it always grew back to the same length. The reason is as the physician mentions, it follows a cycle and then falls out. Note that hair doesn't have any signals running up and down its length to tell it when its at the right length to "stop growing" so to speak, because there is no such thing as a hair that knows how long it is. Hair has no way of determining how long it is so it can't grow to a certain length and stop. Instead, it grows for a certain amount of time and stops and then falls out.

So each individual has a personal maximum hair length they can achieve. There's a Chinese guy with hair 18 feet long. He's a freak. Not because he let it grow that long but because his hair's growth period is so much longer than most people's.